In order to plan a multi-step ovarian cancer screening program it is necessary to have very precise information about the characteristics of the contemplated screening tests. Slight errors in the estimate of the specificity of a single test will lead to large errors in the actual cost of a program and in the number of worrying false positive results. The tumor antigen, CA-125, is of increasing interest as a possible primary screening test for the early detection of ovarian cancer. This project intends to characterize the distribution of values of CA-125 in the population of women over 50 years old. By testing four hundred women once, the precision of the specificity point estimates will be high. By testing four hundred women twice, it will also be possible to make a precise estimate of the intra-individual variability. This information will make it possible to make reliable estimates about the usefulness of repeat testing in a screening program. Four hundred women, 50 years or older, will be recruited from a variety of ambulatory care settings in a medical center. They will each have two venipunctures. Radioimmunoassay for quantitative CA-125 levels will be performed using an inexpensive commercially available kit which would be suitable for use in mass screenings.